Most semiconductor devices require low-resistance contacts that allow the passage of electrical current from the semiconductor to the external circuit or to other semiconductor devices. These contacts, known as ohmic contacts, must be of sufficiently low resistance so as to contribute negligible voltage drop as compared to the voltage drops within the semiconductor device itself. Ohmic contacts to gallium arsenide (GaAs) semiconductor devices are required for the fabrication of field-effect transistors, lasers, light-emitting diodes, photodetectors and many other discrete and integrated devices. High-performance devices, particularly those that have submicron dimensions and are composed of many thin layers of GaAs and related semiconductors such as aluminum-gallium arsenide (AlGaAs) or indium-gallium arsenide (InGaAs), require contacts that are of very low resistance (&lt;1.OMEGA.-mm). Furthermore, such contacts should be shallow and uniform. In other words, the contact metallization should not consume the GaAs in an irregular way. Deep penetrations of contact metal prevent control of the semiconductor layer to be contacted. Still further, the electrical characteristics of the contact should not deteriorate during thermal treatments required for circuit fabrication and packaging steps performed after ohmic contact fabrication. Such processing and packaging steps such as brazing and polyamide curing may require prolonged annealing at temperatures up to 400.degree. C. The ohmic contact must be stable at temperatures at least as high as this annealing temperature.
Ohmic contacts to n-type GaAs that exhibit low contact resistance (0.1-1.OMEGA.-mm) can be fabricated by employing metallizations such as gold-germanium-nickel. Contacts of this type become ohmic during an annealing treatment at a moderate temperature (400.degree.-500.degree. C.), which causes the laterally nonuniform consumption of the GaAs during localized melting of the metallization. The contact interface projects as deep as 0.2 .mu.m into the GaAs substrate. The metallization spreads laterally as well, making these contacts inappropriate for submicron and shallow junction devices.
This uniformity problem has been solved by employing metallizations such as germanium-palladium and silicon-palladium that react uniformly without melting. We and others have disclosed such a solution in three technical articles, a first by Marshall et al entitled "Nonalloyed ohmic contacts to n-GaAs by solid-phase epitaxy of Ge" appearing in Journal of Applied Physics, volume 62, 1987 at pages 942-947, a second by Wang et al entitled "An investigation of a nonspiking Ohmic contact to n-GaAs using the Si/Pd system" appearing in Journal of Materials Research, volume 3, 1988 at pages 922-930, and a third by Marshall et al entitled "Ohmic contact formation mechanism in the Ge/Pd/n-GaAs system" appearing in Materials Research Symposium Society Proceedings, volume 148, 1989 at pages 163-168. Both these types of contacts contain group-IV elements from the fourth column of the periodic table, i.e., germanium, silicon, or tin, which is, we believe, incorporated within or directly upon the gallium arsenide surface as a result of a thermal treatment. It is this Ge:GaAs layer that results in the low contact resistance. Specifically, the germanium may initially occupy gallium sites in the GaAs, thereby donating conduction electrons which makes the potential barrier at the metal-semiconductor interface more transparent to electrons. Alternatively, the germanium can reside in intimate contact with the GaAs, thereby reducing the height of the potential barrier.
The fact that germanium and the other group-IV elements can also occupy the arsenic site in a GaAs crystal, however, results in an inherent instability of contacts based on group-IV elements. Group-IV elements on arsenic sites act as acceptors of electrons, thereby counteracting the effects of the group-IV elements on gallium sites. When a contact based on group-IV elements is annealed at temperatures of 400.degree. C. and above, the compensation of donors by acceptors leads to an increase in the contact resistance.
Earlier proposals for ohmic junctions have involved the use of indium. Wright et al have disclosed an abrupt but ohmic junction between InAs and GaAs in a technical article entitled "In situ contacts to GaAs based on InAs" appearing in Applied Physics Letters, volume 49, 1986 at pages 1545-1547. Marvin et al have disclosed the diffusion of In through an intervening Pt layer to form InGaAs in the GaAs substrate in a technical article entitled "In/Pt ohmic contacts to GaAs" appearing in Journal of Applied Physics, volume 58, 1985 at pages 2659-2661. In a technical article entitled "In/GaAs reaction: Effect of an intervening oxide layer" appearing in Applied Physics Letters, volume 49, 1986 at pages 818-820, Ding et al have emphasized the epitaxial crystallinity of the InGaAs regions formed by coating GaAs with an indium layer.
Other proposals have involved the use of palladium. One of the present inventors and others have discussed the chemical reactions at the palladium/gallium-arsenide junction in a technical article by Sands et al entitled "Initial stages of the Pd-GaAs reaction: Formation and decomposition of ternary phases" appearing in Thin Solid Films, volume 136, 1986 at pages 105-122.
Yet further proposals have involved both indium and palladium. Allen et al have disclosed such an ohmic contact in a technical article entitled "Ohmic contacts to n-GaAs using In/Pd metallization" appearing in Applied Physics Letters, volume 51, 1987 at pages 326-327. Allen et al first evaporated on the GaAs substrate a 40 nm palladium layer and then a 400 nm In layer. A 500.degree. C. anneal then produced a GaAs/In.sub.3 Pd/In structure. The authors nonetheless believed that InGaAs was formed between the GaAs and the In.sub.3 Pd. The present inventors and another have discussed the mechanics of the Allen et al article in a technical article entitled "Solid-phase regrowth of compound semiconductors by reaction-driven decomposition of intermediate phases" appearing in Journal of Materials Research, volume 3, 1988 at pages 914-921. In that article we considered both combinations of palladium-germanium and palladium-indium and concluded that they produced ohmic contacts to GaAs by somewhat different mechanisms.
Murakami et al have disclosed low resistance, thermally stable contacts in U.S. Pat. No. 4,796,082 and in a technical article entitled "Thermally stable, low-resistance NiInW ohmic contacts to n-type GaAs" appearing in Applied Physics Letters, volume 51, 1987 at pages 664-666. These ohmic contacts are fabricated by employing a combination of indium and a transition metal such as nickel, palladium, or platinum. The indium reacts with GaAs to form a layer of InGaAs. This layer of InGaAs, sandwiched between layer of the metal and a GaAs substrate, produces a low resistance contact as a result of the reduced potential barriers, an effect that is well known to those skilled in the art. However, the Murakami contacts suffer several disadvantages.
Such contacts are only stable if the ratio of transition metal to indium is sufficient to completely consume the indium and other low-melting-point phases during thermal annealing. If unreacted indium remains in the contact, a portion of the contact will melt whenever the contact temperature exceeds 156.degree. C., the melting point of indium. Such repeated melting prevents the application of such contacts in submicron devices.
Indium-based contacts that contain only a small amount of indium are stable at high temperatures. Murakami et al require that the atomic percentage of indium be less than 70%. However, low-indium contacts must be annealed at temperatures above 700.degree. C. to attain low values of contact resistance. The incongruent evaporation point for GaAs is about 650.degree. C. in vacuum. Above these temperatures, As preferentially evaporates, leaving globules of Ga. To prevent this catastrophic decomposition of the GaAs substrate, Murakami et al cap the substrate with a refractory material such as silicon nitride. Alternatively, the contact could be annealed in a hazardous arsenic vapor ambient. Both alternatives add extra steps and expense to the processing of GaAs devices and circuits. Furthermore, the high temperatures result in the agglomeration of the metallization, with penetrations of the metallization as deep as 70 nm into the GaAs substrate. Murakami et al prevent the metallization from becoming electrically discontinuous by depositing a thin metallic refractory layer on the transition-metal/indium contact before annealing. Although this capping layer maintains the electrical continuity of the metallization, it must be kept thin (&lt;10 nm) to allow standard lift-off patterning. As a result of the limited thickness of the refractory metal cap, the sheet resistance of the annealed metallization is unacceptably high (.about.10.OMEGA./.quadrature.). To reduce the sheet resistance by a factor of five to ten, an additional layer of a low resistivity metal such as gold must be deposited after contact annealing. These extra processing steps add to the complexity and expense of GaAs device and circuit fabrication.